I usually approach girls on the stairmaster/stationary bike
you got to get her without her headphones on. Ideally when shes first getting on the machine. I open most girls with this :
ME : (looking her up and down with a cocky smile) What do you want to race me or something?
HB : (giggles) Maybe
ME : Well im probably the best stationary bike rider you have ever met...im like the Lance Armstrong of spin class so you better bring your A game (cocky funny)
HB : (Will either give an IOI or a DOI)
Then your pretty much in to open her with other routines.
If a girl is working out with free weights or doing ab work I pretty much will never approach them....its creepy IMO the stairmaster and the bike are just more casual gym exercises that are far easier to initiate conversations with.
hopefully that helps you out a little
As for the "meathead pretty body" comment...get over yourself. If you think women dont look at guys who take care of their bodies by exercising regularly different than your average slightly overweight tool then you have another thing coming.
Ill be honest I like to work out for self confidence and to convey a look that i take care of my body, it reflects the type of person I am.